Attention-Deficit/Hyperactivity Disorder (ADHD) is one of the most common disorders in pediatric psychiatry affecting 3-8% of school age children. Although ADHD is primarily characterized by inattention, hyperactivity and impulsivity, emotional lability is a common and impairing symptom in this population. Little is known, however, about the neurobiological substrates that underlie emotional lability in children with ADHD. Moreover, psychostimulants, the first-line treatment of ADHD, are known to reduce ADHD symptoms including emotional lability, yet the neurobiological mechanisms by which psychostimulants have their salutary effects upon emotional lability have scarcely been studied. To address this knowledge gap, we propose to conduct a longitudinal, multimodal MRI study to examine the effects of psychostimulants on the brain's affective network in children with ADHD. We will combine a multimodal MRI study with a randomized clinical trial of the psychostimulant lisdexamfetamine such that study participants will have an MRI scan prior to treatment and then a second MRI scan after 12 weeks of treatment with either lisdexamfetamine or placebo. The study will advance our knowledge of the neurobiology underlying emotional lability in children with ADHD, as well as the mechanism by which psychostimulants alleviate this symptom. Advancing this type of knowledge is critical for several reasons, but most importantly a clearer understanding of the neurobiological mechanisms by which psychostimulants are effective will promote the development of more effective and targeted treatments, as well as advance our understand of the pathophysiology of ADHD.